The present invention relates to a rubber-based product, a method for producing the product and to a method for reducing the rolling resistance of a tire formed from said product.
One of the main concerns of tire manufacturers is to increase the life of tires. In particular, it is important to increase endurance, with regard to oxidation of the rubber compositions, the metal or textile reinforcements and the interfaces between the rubber compositions and these reinforcements in tires.
One known process for reducing such oxidation phenomena involves restricting the quantity of oxygen, which arrives in a zone of the tire which is particularly sensitive to oxidation, whether in the inflation air or the outside air. Thus, a butyl rubber layer, which is impermeable to oxygen, has, for a long time, been used against the inner wall of tires by manufacturers. Unfortunately, the butyl rubber is not totally oxygen-impermeable, and cannot prevent oxidation of the tire even from reduced oxygen flow during long term use.
Other materials which are more airtight than butyl rubber have been proposed to reduce tire oxidation as described, for example, in U.S. Pat. No. 5,236,030, U.S. Pat. No. 4,874,670, U.S. Pat. No. 5, 036,113, EP-A-337 279, U.S. Pat. No. 5,040,583 and U.S. Pat. No. 5,156,921. However, these materials are expensive and their use in tires causes many problems.
Another process for avoiding the problems of oxidation involves chemically trapping the oxygen by accelerated thermo-oxidation of a rubber composition which acts as a buffer, arranged between a main oxygen source and the zone which it is desired to protect from oxidation. By way of example, such a buffer composition may be located between the inner face of the tire, which may possibly be covered with butyl rubber, and the carcass ply, in order to reduce the quantity of oxygen which comes into contact with this ply from the inflation air. Such an arrangement is seen, in particular, in tires intended to be mounted on heavy vehicles. In order to accelerate the fixing of the oxygen, these buffer compositions comprise a metal salt which catalyzes oxidation, in particular a cobalt salt. The effect of this salt is to activate homolytic decomposition of the hydroperbxides produced upon aging resulting from the afore-mentioned oxidation phenomena. The metal salt is preferably introduced in an amount of 0.2 to 0.3 parts by weight (xe2x80x9cphrxe2x80x9d) in equivalent of cobalt to 100 parts by weight of the elastomer in the buffer composition. The amount of oxygen which can be trapped by this buffer composition is increased by about 50 to 100%, compared to the same composition without cobalt salt. Unfortunately, however, this improvement with regard to oxidation has been shown to be accompanied by a major increase in hysteresis losses of the buffer composition resulting from the significant quantity of cobalt salt introduced in the elastomer. This increase in hysteresis losses results, first, in an undesired self-heating of the composition, which results in a shortened life. Second, there is an undesired increase in the rolling resistance, which is contrary to the aim of tire manufacturers to restrict rolling resistance as far as possible in order to reduce fuel consumption.
For these reasons, the use of buffer compositions, no matter how attractive, has not been developed as much as expected.
European Patent application EP-A-507 207 describes a method for trapping oxygen by means of a buffer elastomeric composition, which is contained in a layer of a package. This buffer composition is in particular characterized by the presence of a transition metal salt designed to activate oxygen fixation. As above mentioned, the metal salts that are preferably described therein are cobalt salts. Other metals such as manganese or even iron are contemplated, but not in relation to specified salts.
The present invention is based on the unexpected finding that the above described disadvantages can be greatly reduced by using specific iron salts, instead of cobalt salts, in antioxidation buffer compositions, to produce a rubber-based product for use in tires. The buffer compositions comprising such iron salts are used to provide a buffer zone to trap the oxygen external to said product in order to protect at least one sensitive zone of the rubber based product from oxidation. The buffer compositions comprise a composition containing at least one elastomer which comprises at least one iron (III) salt intended to activate the oxidation in said composition. More precisely, the elastomer composition comprises an iron salt selected from iron (III) acetylacetonate and an iron (III) salt of a carboxylic acid having the formula
Fe(CnH2nO2)3 in which n is between 6 and 23.
Iron (III) salts of carboxylic acids which correspond to this formula include salts of hexanoic, 2-ethylhexanoic, lauric, tridecanoic, myristic, pentadecanoic, palmitic, heptadecanoic, stearic, nonadecanoic, eicosanoic, heneicosanoic, docosanoic and tricosanoic acids. Preferred salts include iron (III) acetylacetonate, iron (III) hexanoate, iron (III) 2-ethylhexanoate, or iron (III) laurate.
According to a further embodiment of the invention, the iron salt is an iron (III) salt of a fatty acid having the formula
Fe(CnH2nO2)3 in which n is between 13 and 23.
Preferably, said iron (III) salt is iron (III) stearate or iron (III) palmitate.
Preferably, the quantity of the iron (III) compound in the composition comprises from 0.01 to 0.02 phr of equivalent in iron, wherein xe2x80x9cphrxe2x80x9d means parts by weight of iron (or other metal) equivalent to 100 parts by weight of the elastomer or of all the elastomers present in the composition.
The composition according to the invention is based on natural or synthetic rubber, or a blend of two or more of these rubbers. Synthetic rubbers capable of being used in the composition according to the invention include diene rubbers such as polyisoprene, polybutadiene, mono-olefinic rubbers such as polychloroprene, polyisobutylene, styrene-butadiene or styrene-butadiene-isoprene copolymers, acrylonitrile-butadiene-styrene copolymers and ethylene-propylene-diene/terpolymers.
Preferred synthetic rubbers include the diene rubbers, in particular any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms, any copolymer obtained by copolymerization of one or more conjugated dienes with one another or with one or more vinyl aromatic compounds having 8 to 20 carbon atoms.
Suitable conjugated dienes are, in particular, 1,3-butadiene, 2-methyl-1,3-butadiene, the 2,3-di(C1 to C5 alkyl)-1,3-butadienes such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Suitable vinyl aromatic compounds are, in particular, styrene, ortho-, meta-and para-methylstyrene, the commercial mixture xe2x80x9cvinyl-toluenexe2x80x9d, para-tertiobutylstyrene, the methoxy-styrenes, the chloro-styrenes, vinyl mesitylene, divinyl benzene and vinyl naphthalene.
The copolymers comprise between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinyl aromatic units. The elastomers may have any microstructure, which is a function of the polymerization conditions used, in particular of the presence or absence of a modifying and/or randomizing agent and the quantities of modifying
and/or randomizing agent used. The elastomers may, for example, be block, statistical, sequenced or microsequenced elastomers, and may be prepared in dispersion or in solution.
Preferred synthetic diene rubbers are polybutadienes, particularly those having a 1,2 bond content of between 4% and 80% and those having a content of cis-1,4 bonds of more than 90%, polyisoprenes; butadiene-styrene copolymers, and in particular those having a styrene content of between 5% and 50% by weight and, more particularly, between 20% and 40% by weight, a 1,2 bond content of the butadiene part of between 4% and 65%, and a content of trans-1,4 bonds of between 30% and 80%; those having a total content of aromatic compounds of between 5% and 50% by weight and a glass transition temperature (Tg) of 0xc2x0 C. to xe2x88x9280xc2x0 C., and in particular those having a styrene content of between 25 and 35% by weight, a vinyl bond content of the butadiene part of between 55% and 65%, a content of trans 1,4 bonds of between 20 and 25%, and a Tg of between xe2x88x9220xc2x0 C. and xe2x88x9230xc2x0 C.
In the case of butadiene-styrene-isoprene copolymers, suitable polymers include those having a styrene content of between 5% and 50% and, more particularly, between 10% and 40%, an isoprene content of between 15% and 60% by weight, and more particularly between 20% and 50%, a butadiene content of between 5% and 50% by weight, and more particularly between 20% and 40%, a 1,2 bond content of the butadiene part of between 4% and 85%, a content of trans-1,4 bonds of the butadiene part of between 6% and 80%, a content of 1,2 plus 3,4 bonds of the isoprene part of between 5% and 70%, and a content of trans-1,4 bonds of the isoprene part of between 10% and 50%.
The synthetic rubbers may be coupled and/or starred or alternatively functionalized with a coupling and/or starring or functionalizing agent. The rubbers may be vulcanized and/or reticulated by any known agents including, inter alia, sulphur, peroxides, and bis-maleimides.
The composition according to the invention comprises conventional fillers and additives, including, inter alia, carbon black, silica, stearic acid, reinforcing resins, silicic acid, zinc oxide, activators, pigments, vulcanization accelerators or retarding agents, anti-ageing agents, anti-reversion agents, antioxidants, oils or various working agents, tackifying resins, adhesion promoters to metal, antioxidant waxes, bonding agents and/or covering agents for silica.
The compositions according to the invention may be used in a very wide variety of applications and for numerous rubber products, in particular, tires, as buffer compositions between an oxygen source, in particular the inflation air or external air, and a zone to be protected in the tire. For example, these compositions can be used within the inner rubber, between the inner rubber and the carcass ply, between the carcass ply and the crown plies, between the crown plies and the tread, between the carcass ply and the sidewall, or even on the outside of the sidewall.
The iron salts according to the invention are used very differently from the known uses of iron compounds in the rubber industry, such as, for example, the known use of oxidizing salts to promote mastication of rubbers (peptizing properties) or devulcanization for recycling, such as described, for example, in U.S. Pat. No. 3,324,100, EP-A-157 079 and RU-A-2 014 339.
The method for producing a rubber-based product according to the present invention, involves incorporating, by mechanical working, said iron (III) salt in to the elastomer or elastomers in the composition, in order to produce the buffer zone composition.
In a further embodiment of the invention, the iron (III) salt is incorporated into the elastomer composition at the same time as a filler intended to reinforce the composition.
The present invention also provides a method for reducing the rolling resistance of a tire by incorporating by mechanical working an iron (III) salt as defined above into the one or more elastomers used in a composition forming the tire.
The invention will be readily understood with the aid of the following non-limiting examples.
These examples are either examples in accordance with the invention or examples which are not in accordance with the invention. In the latter case, the compositions comprise either no metallic compound or they comprise cobalt salts or iron compounds not included in the group of iron (III) salts according to the invention.
The effectiveness of the iron or cobalt compounds to activate oxidation in the composition was evaluated by subjecting the compositions to thermo-oxidizing aging. The oxygen uptake was measured by elemental analysis and the evolution of mechanical properties of the compositions, such as the modulus, the hysteresis loss and the breaking properties, was determined.
The tests were carried out under the following conditions:
Unless indicated otherwise, all the tests were carried out on samples that had been vulcanized by curing for 10 minutes at 150xc2x0 C.
A ventilated oven at temperatures of 65xc2x0 C. and 85xc2x0 C. was used. These temperatures were considered as representative of the temperature range during the operation of tires.
Hysteresis losses, or hysteresis (HL) are a loss of energy measured at 60xc2x0 C. by rebound at the sixth impact relative to energy supplied. The value, expressed in %, is the difference between the energy supplied and the energy restored, relative to the energy supplied. The deformnation for the losses measured was 40%.
The moduli of elongation at 10% elongation (M10) and 100% elongation (M 100) were measured in accordance with ISO Standard 37.
The breaking stresses (Bs) were also measured in secant moduli (MPa) and the elongations at break (Eb) in %. All these tensile measurements were effected under normal conditions of temperature and hygrometry in accordance with ISO Standard 37.